1. Field of the Invention
The present invention relates to image communications, and more particularly, to techniques for transmitting the same image to a plurality of terminals under different communication conditions.
2. Description of the Related Art
In recent years, image communications, more specifically, broadcast or multicast of image data has grown in importance in the field of mobile communications. In 3GPP (3rd Generation Partnership Project), the broadcast and multicast of image data have been under investigation for providing MBMS (Multimedia Broadcast and Multicast Service).
Since image data has a larger amount of data than conventional audio data, it is required to efficiently transfer the image data within a limited communication capacity of mobile communications. A conventional system which broadcasts or multicasts image data encodes images using a hierarchical coding scheme for transfer from a base station to mobile terminals of a CDMA (Code Division Multiple Access) mobile communication (see, for example, an article “Tdos MBMS-0033, “Scalable Multimedia Broadcast and Multicast Service (MBMS)”, Samsung Electronics, 3GPP MBMS Workshop, London, UK, May 6-7, 2002).
According to this article, an image is encoded into data comprised of two layers (a base layer and an enhanced layer). Data in the respective layers generated by the encoding are called “base layer data” and “enhanced layer data,” respectively. The base layer data has a high importance level for restoring the image, and the image can be generated even by decoding only the base layer data. However, an image generated only from the base layer data is inferior in resolution to an image generated by decoding both the base layer data and enhanced layer data.
In the system of the foregoing article, the base layer data alone is transmitted with large power so that all mobile terminals within a cell can satisfactorily receive the base layer data. On the contrary, the enhanced layer data is transmitted with power small enough to be satisfactorily received only by mobile terminals under good communication conditions, such as those situated near the base station. This strategy reduces the total transmission power in the system to efficiently utilize the communication capacity in the CDMA communication.
Referring to FIG. 1, a conventional mobile communication system comprises core network (CN) 91, and a radio access network (RAN) 94. Server 95 is connected to CN 91, while user equipment (UE) 96, 97 can be connected to RAN 94. Assume herein that image data is broadcast or multicast from server 95 to UE 96, 97.
CN 91, which is a core of a mobile communication network, typically includes a plurality of switches (not shown) and can set an arbitrary communication route. RAN 94, which is responsible for accesses to the mobile communication network, includes radio network controller (RNC) 92 and base station device (Node-B) 93. Node-B 93 connects with UE 96, 97 over the air within a cell covered thereby to provide a radio channel for communications between UE 96, 97 and a partner device (here, server 95). RNC 92 is responsible for a variety of settings and control for Node-B 93, and for a calling process for establishing communications between UE 96, 97 and the partner device.
Server 95 encodes an image in accordance with an encoding scheme which involves two layers with different importance levels when it broadcasts or multicasts image data to UE 96, 97. This encoding results in generation of base layer data 98 and enhanced layer data 99.
Base layer data 98 and enhanced layer data 99 are sent from Node-B 93 through CN 91 and RNC 92. In this event, base layer data 98 and enhanced layer data 99 are transmitted through physical radio channels different from each other. Also, these radio channels are transmitted with transmission powers different from each other, depending on the importance level of each layer. Base layer data 98 is transmitted with higher transmission power than enhanced layer data 99 because base layer data 98 has a higher importance level.
Upon satisfactory receipt of both base layer data 98 and enhanced layer data 99, each UE 96, 97 displays a high resolution image using both data in decoding. On the other hand, when UE 96, 97 cannot satisfactorily receive enhanced layer data 99 but can satisfactorily receive only base layer data 98, each UE 96, 97 displays a low resolution image by decoding base layer data 98.
In FIG. 1, UE 96 is under good communication conditions, because it is located near Node-B 93, and can therefore satisfactorily receive both base layer data 98 and enhanced layer data 99. On the other hand UE 97 is under bad communication conditions because it is located far from Node-B 93, though located within the cell covered by Node-B 93, and can therefore satisfactorily receive only base layer data 98 which has higher transmission power. As a result, UE 96, under good communication conditions, can display a high resolution image, and UE 97, under bad communication conditions, can also display an image though the resolution is lower than that of the image displayed by UE 96.
As described above, the conventional system has the ability to efficiently utilize the communication capacity by ensuring that an image is displayed even on UE under bad communication conditions, while degrading the resolution of the image, to reduce the total transmission power.
However, the foregoing conventional system has the following problems.
In the conventional system illustrated in FIG. 1, one image is encoded into two data, i.e., base layer data and enhanced layer data which are transferred through different paths from each other, so that the system requires a call control which handles these data in pair. For this reason, extra processing must be added to CN 91, RNC 92, and UE 96, 97.
Also, since base layer data and enhanced layer data of the same image pass CN 91 through different paths, a difference in delay must be absorbed before the image is decoded. Further, although the Internet may be connected to CN 91 through a gateway so that server 95 is connected to the Internet, the difference in delay is even increased in such a configuration. Thus, a buffer for temporality storing data is required for RNC 92 or UE 96, 97 in order to absorb the difference in delay. Furthermore, RNC 92 or UE 96, 97 requires extra processing for synchronizing the data using the buffer.